The production of ethanol by a process based on the "dry milling of corn", comprises enzymatically converting starch in the corn to sugar, then fermenting the sugar to produce a "beer" from which industrial grade ethanol is recovered by distilling the beer. This fermentation process produces, along with the ethanol product, a small quantity of lactic acid (2-hydroxypropanoic acid) and glycerol (1,2,3-propanetriol) as by-products. To date, the solids recovered from the beer are the major by-product which is sold as DDG (distillers dried grain) or DDGS (distillers dried grain solubles). Ethanol production using the dry milling process also generates several other by-products none of which is recovered either individually, or one with another, in combination, because they are produced in too low a concentration (hereafter "conc" for brevity) to make recovery economical. With specific respect to lactic acid and glycerol, to date, the perceived wisdom of running an ethanol plant using the dry milling process required minimizing the formation of glycerol and lactic acid, because the lactic acid in particular, suppressed the formation of ethanol and adversely affected profitability. Moreover formation of lactic acid and glycerol consumes valuable substrate (or fermentable sugars).
Since the economics of increasing ethanol at the expense of depressing the concentrations of lactic acid and glycerol formed, were not inviting, it was decided to separate the lactic acid and glycerol. This process is directed to the recovery of both lactic acid and glycerol, together, from a stream of aqueous solids ("whole stillage") produced from the bottoms of an ethanol fractionation column such as is conventionally used in a corn dry-milling ethanol plant.
More specifically this invention relates to the recovery of both lactic acid and glycerol, together, from "corn thin stillage" (or, "thin stillage" for brevity) discharged from a centrifuge in which relatively large, water-insoluble, solids in the "whole stillage" are separated. In a conventional ethanol plant for the dry-milling process, centrifugally separated solids from the whole stillage are fed to rotary driers where the solids are mixed with syrup produced in evaporators (see flow sheet of conventional plant, FIG. 1) and dried to produce DDG or DDGS. The operation and maintenance of the evaporators are to produce the syrup by removing water from the filtered thin stillage stream. This thin stillage typically contains a substantial concentration of water-insoluble ("suspended") solids, as well as "dissolved" solids, generally, only the solids larger than 10 .mu.m in nominal diameter having been removed. Even with an exceptionally effective centrifuge, only solids larger than 1 .mu.m may be removed. The solids, both suspended ("insolubles.") and dissolved, foul evaporators, so that their operation and maintenance is a major cost of operating a corn dry milling ethanol plant. Even a slight reduction in the costs of operating and maintaining the evaporators produces a dramatic increase in profitability. The term "insolubles" herein refers to material which is water-insoluble.
Because lactic acid and glycerol are present in the thin stillage in relatively very small amounts, together in a conc less than 5% on a weight basis (weight lactic acid and glycerol/weight thin stillage or "w/w"), recovering them has been such a technically unpromising and predictably unrewarding task, that to date, there has been no attempt, in a commercial process, to do so. Though, on a laboratory scale, the separation of lactic acid and glycerol from thin stillage is not an unusually demanding task, there is no record in the prior art of an operable process with the potential, commercially to recover either lactic acid or glycerol, or both together, from thin stillage.
The reason for the failure to provide an economical process appears to have been a widespread misappraisal and misjudgment of the ability of a membrane means to make a critically important separation in the initial filtration step; and, the failure to realize that the essentially identical relatively low molecular weights of lactic acid and glycerol fortuitously allow their separation and recovery, in combination, from an aqueous stream containing proteins and relatively high mol wt (C.sub.10 -C.sub.24) fatty acids, provided the "right" membranes are used in the "right" combination.
A study of the use of microfiltration ("MF") or ultrafiltration ("UF") membranes in prior art filtration of thin stillage and "steep water" (obtained in a process for wet milling corn) streams, discloses that the main reason for their use is to remove insolubles. Such insolubles, which are very finely divided solid particles dispersed in thin stillage, were filtered from the thin stillage prior to evaporating it in an evaporator used in the corn dry-milling process. Since the main concern was to reduce fouling of the evaporator by dispersed solids, there was no evident concern as to what "solubles" may be entrained in the filtered thin stillage.
It will be appreciated that the term "evaporator" is used herein specifically to refer to the unit operation of concentrating thin stillage in a conventional evaporator used in a conventional facility for the production of ethanol based on the dry-milling of corn. The function of such evaporator means is separate and distinct from that of "dryers", typically rotary dryers, used to dry a mixture of centrifuged ethanol stillbottoms (solids) and syrup such as is conventionally produced in the evaporators.
In contrast, the process of this invention uses a UF system including at least one UF module in a first filtration step to remove a major portion of soluble proteins along with the insolubles at such a high recovery that the concentrate produced does not have to be flowed to an evaporator. "Recovery" is defined as the percent by volume (vol %) of permeate removed, based on 100 volumes of feed to the UF system during the period it is in operation.
It is critical in this step-wise filtration process that the permeate recovery be at least 50% in each filtration step, most particularly in the first step comprising ultrafiltering the thin stillage. It will be appreciated that the higher the permeate recovery in the first step, the higher the recovery of lactic acid and glycerol in the overall process. More specifically, it will be appreciated that since there is essentially no rejection (less than about 1% for each, lactic acid and glycerol) a permeate recovery of 80% in the first step results in an 80% yield of lactic acid and glycerol. Stated differently, for 100 gallons (gal) of thin stillage containing 1.5% lactic acid, the yield is 80 gal of UF permeate containing 1.5% lactic acid. An equal conc of lactic acid leaves with the UF concentrate.
Further, if 100 gal of UF permeate containing only 1% lactic acid are flowed to the second step, namely nanofiltration ("NF") in the membrane separation process carried out in the membrane means used herein, then if 75 gal of permeate are recovered (75% permeate recovery), and the rejection of lactic acid is 25%, the concentration of lactic acid in the NF permeate is 0.75%.
If in a final step, the reverse osmosis ("RO") step in this invention, the permeate recovery was as high as 90%, then the overall permeate recovery for the process would be the product of the individual permeate recoveries in each separation step. If, as is desirable, the permeate recovery in the UF first step is as high as 80%, in the NF second step is as high as 75%, and in the RO third step is as high as 90%, overall permeate recovery is 0.8.times.0.75.times.0.9=0.54.
Since under the most desirable conditions, the overall permeate recovery appeared to be unattractive, there was no motivation to probe the advantages of a process in which the thin stillage was ultrafiltered under particularly specified conditions which provide at least 50% recovery in each step. There was no reason to suspect that such a membrane process would dispense with the conventional use of an evaporator for thin stillage, and, as will be explained hereafter, obviate having to cope with the fouling problems which are the basis for disproportionately large maintenance costs.
In the context of this process it is important to note that the terms MF and UF refer to distinctly different membranes for use in separate and distinct filtration operations, and in the first step of the process of this invention, a MF membrane cannot be used in lieu of a UF membrane. Still other membranes used to make separations in this process are a nanofiltration (NF) membrane and a reverse osmosis (RO) membrane. A MF membrane is used to remove very finely divided solids which are insoluble. The range of sizes of pores in a MF membrane are in the range from 0.2 .mu.m (micrometers or microns) to 10 .mu.m. A UF membrane used in the process of this invention necessarily separates particles smaller than 0.2 .mu.m, preferably in the range from 0.005.mu.m-0.1 .mu.m, and, soluble materials based on molecular weight ("mol wt" for brevity), typically in the range from 1000-200,000 Daltons, referred to as "heavies". A nanofiltration ("NF") membrane is semipermeable and non-porous, and provides a further separation based both on mol wt and ionic charge. Molecular weight cut-offs for non-ionized molecules are typically in the range from 150-1000 Daltons, referred to as "lights". For ions of the same mol wt, membrane rejections will increase progressively for ionic charges of 0, 1, 2, 3 etc. for a particular membrane because of increasing charge density (see "Nanofiltration Extends the Range of Membrane Filtration" by Peter Eriksson, Environmental Progress, Vol 7, No 1, pp 58-59, February 1988). A RO membrane is also semipermeable and non-porous, and requires an aqueous feed to be pumped to it at a pressure above the osmotic pressure of the dissolved substances in the water. Because an RO membrane can effectively remove low mol wt molecules &lt;150 Daltons, and also ions from water, RO membranes are commonly used to demineralize water (e.g. for pretreating boiler feedwater, and recovering potable water from brackish water or sea water).
The desirability of recovering lactic acid and glycerol essentially free from proteins and relatively high mol wt (&gt;200 Daltons) organic compounds from the by-product stream has been fueled by a long-felt need for cheap lactic acid. There are a multiplicity of uses of lactic acid in the food, drug and related industries. Lately there has been a growing emphasis on the use of polymers of monomers derived from lactic acid because they are biodegradable. Glycerol is used not only as an intermediate in numerous syntheses but also as a copolymer with lactic acid for biodegradable resins, and in the pharmaceutical industry. The process of this invention is directed to help satisfy that long-felt need.
The possibility of recovering lactic acid and glycerol from the by-product stream by filtration was first recognized by Brian Burris in a paper titled "Cross-Flow Ceramic Membrane Microfiltration for the Clarification of Ethanol Stillbottoms" presented to the International Conference On (Fuel) Alcohols and Chemicals from Biomass in Guadaljara, Mexico (Winter 1989). He reported that microfiltered (MF) permeate was "sparkling" clear and contained insolubles of less than 0.08%. He evaporated the clarified MF permeate and reduced the fouling of the evaporator, at the same time reducing the viscosity of the evaporated stream (syrup). He suggested that the lactic acid and glycerol could be recovered from the syrup so obtained by conventional means.
Burris reported that a large scale test had been conducted in a 10 MM gal/yr fuel grade alcohol distillery which used dry-milling. Although this plant, unlike most other distillers, centrifuged their feed solution prior to distillation, they had to shut down every ten days to two weeks because the stillbottoms' evaporator would "scum up" with solids. This company had previously evaluated a spiral wound ultrafilter system which proved ineffective due to plugging. Also evaluated was a tubular polymeric cross-flow filtration unit which produced good clarity product for long periods of time with minimal cleaning.
The term "cross-flow" refers to flow in which there are three streams--feed, permeate and concentrate. In contrast, a "dead end" or "depth" filter has two streams--feed and filtrate (or permeate). In cross-flow feed flow through membrane channels, either parallel (or tangential) to the membrane surface, is separated into a concentrate (and/or recycle) stream and permeate stream. The recycle stream retains all the particles and large molecules rejected by the membrane. The feed/recycle stream mixture flows through filter channels and may be totally recycled to the membrane module, or partially removed form the system as reject (concentrate). The flow of feed parallel to the membrane surface creates shear forces and/or turbulence to sweep away accumulating particles rejected by the membrane (see Burris, supra).
Burris presented data comparing flux versus average transmembrane pressure readings at two cross-flow velocities; and, also presented flux decline data (which was minimal) during 10x concentration. The data were obtained with a 0.5 .mu.m membrane. Other membranes used were 0.8 .mu.m and 0.2 .mu.m. In all cases the thin stillage was only microfiltered so that solids with a nominal diameter &lt;0.2 .mu.m and `heavies` remained in the MF permeate. Such small solids were insufficient to vitiate the clarity of the MF permeate and greatly improved the operation of the evaporators. The MF step, introduced to clarify thin stillage before it is flowed to the evaporator, is diagrammatically illustrated in phantom outline in FIG. 1 which depicts an otherwise conventional ethanol recovery process based on the dry-milling of corn.
Since Burris' goal was to minimize the cost of maintaining and operating the evaporator, he was not concerned with removing insolubles smaller than 0.1 .mu.m. Nor was he concerned with removing high mol wt solids above 2.times.10.sup.5 Daltons. The purpose for microfiltration was solely to provide sparkling clear, filtered thin stillage for the evaporator. There is no suggestion that the separation of lactic acid and glycerol, from the syrup produced by evaporation of thin stillage, be made by any membrane filtration process.
Therefore there was no motivation to explore the limitations of operating a UF system as a first step for the particular purpose of separating from the thin stillage, not only insolubles &gt;1 .mu.m, but also those insolubles &gt;0.1 .mu.m along with dissolved solids &gt;2.times.10.sup.5 Daltons.
The total solids concentration (including lactic acid and glycerol) in the thin stillage feed is in the range from 5%-12%, more typically in the range from 6%-10%. This thin stillage is ultrafiltered in the first step of the novel process to provide a permeate recovery high enough to yield a concentrate with a total solids conc in the range from at least 15% to as high as 35%, preferably 20%-30%. The combined concentration of lactic acid and glycerol, typically in the range from 1-3%, depends upon the amount produced in the fermentation process. The minimum yield of lactic acid and glycerol recovered in an economical plant operation is about 40% based on the conc of lactic acid in the thin stillage. The recovery is preferably in the range from about 45-70%, and may be as high as 90% if diafiltration is used. In general, the higher the concentration of lactic acid in the thin stillage, the better the recovery. The same is substantially true for the glycerol. Note that the conc of lactic acid and glycerol in the UF permeate is substantially independent of UF recovery because there is essentially no rejection of either by an appropriately chosen UF membrane.